The present invention relates generally to a process for generating hydrogen, and more particularly to a process for reducing the amount of carbon monoxide in process fuel gas in a water gas shift converter, and to high activity water gas shift catalyst systems for use in such a process.
In a fuel cell, electrical power is generated by a chemical reaction. The most common fuel cells involve the chemical reaction between a reducing agent, such as hydrogen, and an oxidizing agent, such as oxygen. In order to be used in such a fuel cell, hydrocarbon fuel must first be converted into a hydrogen-rich stream.
Fuel processing systems to convert hydrocarbon fuel into a hydrogen-rich stream generally include three principal sections: a primary reactor, a water gas shift converter, and a carbon monoxide cleanup system.
In the primary reactor, the preheated fuel/steam/air mixture comes in contact with the reforming catalysts, and the fuel is converted into products including hydrogen, carbon monoxide, carbon dioxide, and methane. Temperatures in this section typically range from 650–800° C. Various types of primary reactors can be used, such as steam reformers, auto-thermal reformers, and partial oxidation reformers.
High levels of carbon monoxide in the feed stream for the fuel cell can poison the anode electrodes of the fuel cell. As a result, the level of carbon monoxide in the process gas must be reduced before the process gas can be sent to the fuel cell. In the water gas shift converter, the products of the primary reactor are placed in contact with one or more water gas shift catalysts. The carbon monoxide reacts with water to produce hydrogen and carbon dioxide, reducing the concentration of carbon monoxide in the process gas. This reaction is known as the water gas shift reaction. Temperatures in the water gas shift converter are generally in the range of 200–600° C.
The final section of the fuel processing system is the carbon monoxide cleanup system. This section is designed to ensure that the hydrogen stream is of suitable quality for use in a proton exchange membrane (PEM) fuel cell (i.e., typically the carbon monoxide concentration is less than 50 ppm). This is usually described as a subsystem because several different types of catalytic, membrane and/or adsorption sections are combined as a unit, usually also including various heat exchangers.
Water gas shift converters are well known. They typically include a chamber with an inlet for the process gas from the primary reactor to enter and an outlet for the process gas to pass to the carbon monoxide cleanup system. There is a catalytic reaction zone between the inlet and the outlet. The catalytic reaction zone includes a catalyst for converting carbon monoxide to carbon dioxide by the water gas shift reaction according to the following equation:CO+H2O→CO2+H2This reaction not only reduces the carbon monoxide concentration, it also increases the carbon dioxide and hydrogen concentrations of the process gas.
Water gas shift catalysts are known. High temperature (400–450° C.) water gas shift catalysts include iron oxide, chromic oxide, and mixtures thereof. Other water gas shift catalysts include copper, zinc, iron, chromium, nickel, and cobalt compositions, as well as platinum, palladium, rhodium, gold, and ruthenium. Noble metals combined with cerium oxide have been used as water gas shift catalysts. However, they have a relatively low level of activity. U.S. Pat. No. 6,455,182 discloses a water gas shift catalyst which includes a noble metal on a support of mixed metal oxides, in which at least two of the oxides are cerium oxide and zirconium oxide. The cerium oxide and zirconium oxide are present in the range of about 50 to 30 mole % zirconium (42 to 23 wt % zirconia) to 50 to 70 mole % cerium (58 to 77 wt % ceria). The patent limits the amount of zirconium to not less than 30 mole % (23 wt % zirconia) so that the zirconium provides enhanced stability to the catalyst, and not more than 50 mole % (42 wt % zirconia) to prevent phases which are only zirconium oxide and/or only cerium oxide.
There remains a need for high activity water gas shift catalyst systems and for methods of using such catalyst systems.